This invention relates to a belt transmission method for transmitting a rotational force of a driving rotary shaft accompanied with a slight variation in angular velocity (speed variation) to a driven-rotary shaft having rotational inertia via a belt, and relates to a device in which the above method is used.
In general, an engine (internal-combustion engine) generates driving energy only in an explosion stroke but no rotational torque in any other strokes, so that its crank shaft generates a variation in angular velocity at any time. Accordingly, in the case that the rotational torque of the engine is transmitted to the other driven rotary shaft through a belt, influences from the variations in angular velocity become more outstanding as the driven rotary shaft is increased in load.
More specifically, in a belt transmission device which is driven by an engine, a variation in angular velocity of the crank shaft synchronizes with a variation in peripheral velocity of the belt. Therefore, when the driven shaft has large rotational inertia, a slip due to the variation in peripheral velocity occurs between a pulley on the driven rotary shaft and the belt. This remarkably shortens a durable life of the belt.
For example, when an alternator as an generator which is one of auxiliaries of an automobile is driven by belt transmission with use of an engine as a driving source, an alternator shaft has large rotational inertia. Thus, the belt repeatedly generates slight slips on the pulley of the alternator shaft due to variations in angular velocity which is typical of the engine. The slips provide various kinds of troubles such as abrasion of the contact surface, heat generation due to friction, generation of noise and the like. Further, since the alternator shaft is generally provided with a pulley larger in diameter than that of the crank shaft of the engine to have a velocity ratio of increasing in velocity with respect to the crank shaft, the above-mentioned troubles become more outstanding.
In particular, if the belt is a V-ribbed belt to be applied for the purpose of compaction of its occupying space, the abrasion of the belt surface directly results in remarkably shortening the durable life of the V-ribbed belt. In this case, prevention of the slips is impossible.
To deal with the above troubles, common engines employ a method of providing a flywheel on the crank shaft to increase its inertial force so that a smooth running may be accomplished. However, since the crank shaft has a limitation in its torsional strength, the increase in inertial force is also limited. It cannot be avoided that in a gasoline engine, its crank shaft generates variations in angular velocity of about 1.5 to 2.0.degree. at maximum, and in a diesel engine, its crank shaft generates variations in angular velocity of about 6.degree. to 8.degree. at maximum.
Further, in order to improve a transmission belt in its structure and strength, various kinds of attempts have been conventionally made thereby obtaining certain effects. However, as mentioned above, since it is impossible to completely avoid variations in angular velocity of the driving rotary shaft, reduction in abrasion of the belt and reduction in generation of sounds by the above conventional attempts mean only that they have been performed to a certain extent.
To solve the problem, Applicant has previously proposed, in a belt transmission technique accompanied with such variations in angular velocity of the rotary shaft, a device in which a one-way clutch is interposed between the rotary shaft and a pulley. According to this device, power transmission from the driving rotary shaft only in a normal direction is performed by a connecting operation of the one-way clutch and a rotational force in a relatively reverse direction is broken by a disconnecting operation of the one-way clutch, so that it is prevented that an excessive stress acts on the belt thereby accomplishing an extended belt life and reduction in sound (See Japanese Patent Application Laid-Open Gazette No.61-228153).
However, the proposed device also has some problems. In detail, when the proposed belt transmission device having the one-way clutch is used in order to drive, for example, auxiliaries of the automobile, a required frequency for the one-way clutch is 23 to 26 Hz in an idle rotational area of 700 to 800 rpm of a 4-cycle 4-cylinder engine while a required frequency for the one-way clutch is 186 to 200 Hz in a maximum rotational area of 5600 to 6000 rpm of the engine.
On the other hand, when the one-way clutch is a roller type one, its allowable frequency is at most 60Hz. Accordingly, though no trouble occurs in the idle rotational area of the engine, the frequency of the one-way clutch exceeds the allowable frequency at the time of high-speed rotation thereby presenting breakage of a preload spring of the clutch or the like. In other words, the above-proposed belt transmission device has disadvantages in durability and reliability of the one-way clutch.
The present invention has been made in view of the foregoing problems and has its object of enhancing the durability of a one-way clutch while accomplishing reduction in noise and an extended belt life by the one-way clutch in such a manner as not to transmit to the one-way clutch a variation in angular velocity at a high frequency exceeding the allowable frequency of the one-way clutch.